Analysis of Magnetic Helicity Injection by RF Waves

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The mechanism of magnetic helicity injection and current drive by RF waves was studied both analytically and numerically.1. Current drive via non-resonant interaction between RF waves and plasma studied in a stationary state. The non-resonant interaction generates a non-conservative force, in addition to the usual conservative ponderomotive force. The force of our concern mainly acts as an internal force among plasma species ; the net momentum input from the wave to the plasma is small in this process. The current drive by this process is different from the conventional RF current drive schemes based on resonant wave-particle interactions, and is associated with the change of RF wave helicity.2. A one-dimensional wave code was extended to calculate the driven current associated with the nonresonant force. The analysis is applied to ICRF waves in a large tokamak. The profiles of driven current and helicity absorption were obtained. It was confirmed that the local current drive efficiency is independent of the plasma density and strongly depends on the temperature. The global current drive efficiency, however, depends on central density owing to the change of propagation and absorption structure.3. Current drive by the helicity injection via a plasma surface requires spatial diffusion of the current. In order to study the mechanism of the current diffusion in a tokamak, we have developed a three-dimensional bounce-averaged Fokker-Planck code. This code was used to simulate the electron cyclotron current drive experiment. The broadening of the current profile and the reduction of the current drive efficiency have been observed by including spatial diffusion of fast electrons.